Chicken 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA reductase), the rate limiting enzyme in cholesterol biosynthesis, has been purified to homogeneity from liver microsomes by agarose-blue dextran chromatography. The enzyme was a single band on electrophoresis, and had a monomer molecular weight by SDS electrophoresis of 18,000. A monospecific antibody to chicken HMG-CoA reductase was developed in goats. The chicken antibody demonstrated cross reactivity with rat liver, but not human fibroblast HMG-CoA reductase. Chicken liver HMG-CoA reductase solubilized from liver microsomes could be rapidly inactivated by ATP-Mg ion suggesting that phosphorylation of the enzyme was associated with loss of enzymatic activity. Treatment of the modified enzyme with a cytosolic fraction rich in phosphatase activity rapidly restored enzyme activity. These results are consistent with the concept that phosphorylation-dephosphorylaton may be an important cellular mechanism for control of enzyme activity.